The aerobic cleavage of carbon-carbon double bonds of alkenes, especially to aldehydes, is a valuable transformation of synthetic importance. The use of renewable resources such as unsaturated fatty acid derivatives as feedstock substrates for such reactions is also enticing. High yields of carboxylic acids from disubstituted alkenes may be obtained using reagents such as permanganate/periodic acid, ruthenium tetraoxide with hypochlorite or periodate, and hydrogen peroxide with peroxotungstate catalysts under acidic conditions through formation of epoxide and diol intermediates. The strong oxidizing conditions used in these reactions typically prevent the selective preparation of aldehydes from disubstituted alkenes. For such transformations a common procedure is to utilize the 1,3-dipolar addition of ozone to a double bond to yield an initial ozonide or 1,2,3-trioxolane, which is then decomposed to yield aldehydes under reducing conditions. The explosive nature of ozone, often renders a multistep reaction cascade of epoxidation and hydrolysis to yield a glycol, which can then be cleaved by any number of procedures, as a more attractive alternative. Classically, periodic acid or lead acetate have often been used for the cleavage of glycols to aldehydes, but catalytic methods have also been disclosed including use of oxygen as oxidant.
It would be advantageous to use molecular oxygen, i.e., O2, in a one-step oxidative cleavage of carbon-carbon bonds to yield aldehydes and/or ketones. However, the reaction of hydrocarbons with ground state O2 ubiquitously occurs via free radical autooxidation mechanisms typically initiated by the cleavage of a carbon-hydrogen bond and formation of a carbon centered free radical, for example at the allylic position of an alkene. These mechanistically very complex autooxidation reactions, which involve the interplay of many reactions steps and multiple pathways, result in non-selective product formation, the bane of organic synthesis.
Paramagnetic nitrogen dioxide, NO2, is a known reagent for nitration, nitrosation, and the facilitation of halogenation reactions; but also has been used for oxidation. The most reported oxidation reaction has been that of thioethers and other sulfur containing compounds. Typically, NO2, a “weak” radical, is only active in hydrogen atom transfer reactions when the carbon-hydrogen bond is weak as in alcohols, although electron transfer oxidation of reactive substrates such as anthracene and polyhydroxyarenes is also known. As pertains to the reaction of NO2 with alkenes, commonly addition to the double bond to yield nitroalkenes was observed. In certain cases, epoxidation has been observed such as with diadamantylidene and related hindered alkenes, and the reaction of phenylethenes with NO2 has been reported to yield some carbon-carbon bond cleavage products, although the selectivity and yields are low.
To the Applicant's best knowledge, the selective cleavage of carbon-carbon double bonds with metal-nitro or metal-nitrito complexes has not been reported. In fact, metal-nitro and metal-nitrito compounds have been used only relatively rarely as oxygen donors. Thus, palladium-nitro catalysed oxidation of alkenes and iron heme and non-heme sulfoxidation of thioethers are the most reported transformations. Recently, nitrites have also been used as oxygen donors in the anti-Markovnikov palladium catalysed Wacker reaction.
There remains a need for efficient methods for preparing aldehydes and ketones by carbon-carbon bond cleavage of alkenes, which methods are amenable for industrial applications, as well as the use of renewable carbon sources as substrates.